Semiconductor integrated circuits (ICs) contain individual devices that are typically coupled together using metal line interconnects and various contacts. In many applications, the metal lines are formed on a different level than the devices, separated by an intermetal dielectric, such as silicon oxide or borophosphosilicate glass (BPSG). Commonly used metal lines include aluminum, tungsten and copper, as well as combinations of these materials with refractory metals and silicon. Interconnects used to electrically couple devices and metal lines are formed between the individual devices and the metal lines. A typical interconnect is composed of a contact hole (i.e. opening) formed in an intermetal dielectric layer over an active device region. The contact hole is often filled with a metal, such as aluminum or tungsten.
Interconnects often further contain a diffusion barrier layer sandwiched between the interconnect metal and the active device region at the bottom of the contact hole. Such layers prevent intermixing of the metal and the material from the active device region, such as silicon. Reducing intermixing generally extends the life of the device. Passive titanium nitride (TiN) layers are commonly used as diffusion barrier layers. An example may include the use of titanium nitride interposed between a silicide contact and a metal fill within a contact hole. Further uses of diffusion barrier layers may include a barrier layer interposed between a polysilicon layer and a metal layer in a gate stack of a field effect transistor.
Titanium nitride is a desirable barrier layer because it is an impermeable barrier for silicon, and because it presents a high barrier to the diffusion of other impurities. Titanium nitride has relatively high chemical and thermodynamic stability and a relatively low resistivity. Titanium nitride layers are also often used as adhesion layers, such as for tungsten films. While titanium nitride can be formed on the substrate by physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques, CVD is often the technique of choice.
CVD is a process in which a deposition surface is contacted with vapors of volatile chemical compounds, generally at elevated temperatures. The compounds, or CVD precursors, are reduced or dissociated at the deposition surface, resulting in an adherent coating of a preselected composition. In contrast to physical deposition, CVD does not require high vacuum systems and permits a wide variety of processing environments, including low pressure through atmospheric pressure, and is an accepted method for depositing homogeneous films over large areas and on non-planar surfaces.
CVD is often classified into various types in accordance with the heating method, gas pressure, and/or chemical reaction. For example, conventional CVD methods include cold-wall CVD, in which only a deposition substrate is heated; hot-wall CVD, in which an entire reaction chamber is heated; atmospheric CVD, in which reaction occurs at a pressure of about one atmosphere; low-pressure CVD (LPCVD) in which reaction occurs at pressures from about 10−1 to 100 torr; and plasma-assisted CVD (PACVD) and photo-assisted CVD in which the energy from a plasma or a light source activates the precursor to allow depositions at reduced substrate temperatures. Other classifications are known in the art.
In a typical CVD process, the substrate on which deposition is to occur is placed in a reaction chamber, and is heated to a temperature sufficient to drive the desired reaction. The reactant gases containing the CVD precursors are introduced into the reaction chamber where the precursors are transported to, and subsequently adsorbed on, the deposition surface. Surface reactions deposit nonvolatile reaction products on the deposition surface. Volatile reaction products are then evacuated or exhausted from the reaction chamber. While it is generally true that the nonvolatile reaction products are deposited on the deposition surface, and that volatile reaction products are removed, the realities of industrial processing recognize that undesirable volatile reaction products, as well as nonvolatile reaction products from secondary or side reactions, may be incorporated into the deposited layer. Integrated circuit fabrication generally includes the deposition of a variety of material layers on a substrate, and CVD may used to deposit one or more of these layers.
As an example, one LPCVD process combines titanium tetrachloride (TiCl4) and ammonia (NH3) to deposit titanium nitride. However, LPCVD titanium nitride using these precursors has a tendency to incorporate a large amount of residual ammonium chloride in the film. This residual ammonium chloride detrimentally effects the resistivity and barrier properties of the titanium nitride layer. Once exposed to air, the residual ammonium chloride will cause the titanium nitride layer to absorb water and to form particles, both undesirable effects. It is known that residual ammonium chloride can be reduced by the use of ammonia post-flow, or annealing in an ammonia atmosphere, subsequent to deposition. However, such post-processing leads to reduced throughput and a higher risk of particle formation. It is also known that increased reaction temperatures can be used to reduce the incorporation of residual ammonium chloride. However, this, too, is detrimental as increased processing temperatures reduce the thermal budget available for subsequent processing and often lead to undesirable dopant diffusion.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods of chemical vapor deposition.